The present invention relates to a level probe, hereafter level detecting system, for a powder container in a powder spray-coating apparatus.
The state of the art contains the following:
EP 0 185 479 B1,
EP 0 452 635 B1,
DE 198 08 765 A1,
DE 196 11 533 A1,
DE 39 05 236 A1.
The European patent document 0 185 479 B1 discloses powder spray-coating apparatus for spray-coating objects using a powder coating material, wherein a first pump pneumatically conveys coating powder from a first powder container into a second powder container and a second pump conveys the powder from the second powder container to a spray apparatus. A powder overflow device keeps the powder level constant in the second powder container and returns excess powder into the first powder container.
Both powder containers are fitted with a perforated upper bottom crossed from below to top by the fluidizing compressed air into the powder present on this upper bottom and xe2x80x9cfluidizingxe2x80x9d this powder, that is, the powder particles are kept floating in this fluidizing compressed air.
The European patent 0 452 635 discloses a pneumatic fluidizing and evacuation unit which can be immersed into a powder container in order to fluidize powder therein and to evacuate this fluidized powder from the container and convey it to a spray apparatus or another. The fluidizing and evacuation unit is fitted with a level detector generating an electrical signal as a function of depth of immersion into the fluidized powder. The electrical signal controls an advance system by means of which the fluidizing and evacuation unit jointly with the level detector always is kept at the same depth of immersion in the fluidized powder and is made to automatically follow the dropping powder level when the quantity of powder decreases. As a result, identical powder concentration and identical powder/airflow conditions are maintained at the lower, powder-aspirating end of a suction tube of the fluidizing and evacuation unit regardless of the height of the powder in the powder container. Consequently uniform power feed to a spray apparatus is assured.
It is known in practice to use so-called capacitive detectors as the level detectors, their capacitance changing as a function of the powder concentration in the detector vicinity. This procedure however entails the substantial drawback of spurious test results caused by powder particles which over the time collect at the detector and adhere to it.
The objective of the invention is to solve the problem of designing a level detecting system for powder containers in such manner that it shall operate continuously and accurately regardless of any powder particles adhering to it. Preferably such a level detecting system may be designed to operate not only vertically, but instead in any arbitrary position, for instance also horizontally, when detecting the powder level in a powder container.
The expression xe2x80x9cpowder containerxe2x80x9d not only includes a xe2x80x9crigidxe2x80x9d container, but also a xe2x80x9cflexiblexe2x80x9d one such as a pouch made of plastic or another material.
The goal of the invention is attained using an inductive proximity sensor, further a membrane configured opposite this proximity sensor and moved in reciprocating manner by the pressure exerted by fluidized powder on its external side away from the proximity sensor, and a metallic element mounted on the membrane inside surface opposite the proximity sensor and jointly moving with latter and thus affecting the proximity""s sensor magnetic properties.
The following advantages in particular are offered by the invention: the test signals are permanently accurate regardless of any adhering particles. Accordingly it is also maintenance-free. Moreover it may be used in any arbitrary direction rather than only vertically, namely for instance also horizontally, when determining the powder level in a powder container. It may be assembled from a few, commercially accessible parts and accordingly both its manufacture and its operation are economical.